In circuits for high frequency applications, mobility of charge carriers in the channels of the devices must be made as high as possible. The most successful method to increase the charge carrier mobility is by modulating doping to provide charge carriers in the channel of a high electron mobility transistor, HEMT. Using HEMTs based on multi-layer structures of compound semiconductors such as AlGaAs—InGaAs—GaAs, high quality millimeter devices and circuits have been developed.
A HEMT (100) structure as shown in FIG. 1(a) is constructed on a substrate (101) having a channel layer (102). In order to achieve modulation of conductance of the channel layer (102), a gate (105) of a head length (105Lh) and a stem length (105Ls) is fabricated between a source (103) and a drain (104). To reduce the unwanted parasitic series resistance, the contacts between the source (103) and channel layer (102) and that between the drain (104) and the channel layer (102) are made to have near ohmic behavior. Whereas, the contact between the gate (105) and the channel layer (102) is non-conducting or rectifying in order to achieve modulation of charge density in the channel layer (102). Therefore, the construction of a HEMT will require having at least two metal deposition/patterning processes to serve the two needs. Due to the sensitivity of the InGaAs-based channel layer to photoresist developer and etchants of metals for source, drain and gate, the fabrication of HEMTs is achieved only by a liftoff process. As shown in FIG. 1(b), a first photoresist (110) with a thickness (110t) is applied, baked, exposed through a first mask and developed to form two cavities (113s, 113d). The cavity for the formation of the source (113s) has a top width (113swt) and a bottom width (113swb) and the cavity for the drain (113d) has a top width (113dwt) and a bottom width (113dwb). A first metal layer of a thickness (103t, FIG. 1(c)) is then deposited to form the source (103) and the drain (104) with also a metal layers (103a) in the surrounding areas on the photoresist (110). After the source and drain metal deposition, metal layers (103a) in the surrounding areas are lift-off by immersing the substrate in a solvent which dissolves the photoresist. The isolated source (103) with a width (103w) and the drain (104) with a width (104w) are obtained (see FIG. 1(d)). In order to obtain successful liftoff, the two cavities (113s, 113d) must have reentrant cross-section so that the bottom width (113swb and 113dwb) is substantially larger than the top width (113swt and 113dwt) in FIGS. 1(b) and 1(c). Furthermore, thickness of photoresist (110t) should be substantially larger compared to source and drain thickness (103t).
After the formation of source (103) and drain (104), a second photoresist layer (120) of thickness (120t) is applied, baked, exposed through a second photomask and developed to form a stem cavity (120c) of a width (120w). A third photoresist layer (121) of thickness (121t) is then applied, baked, exposed through a third photomask and developed to form a head cavity (121c) with a top width (121 wt) and a bottom width (121wb). As shown in FIG. 1(e), the bottom width (121wb) of the head cavity (120c) must be made substantially larger than the top width (121wt) to facilitate subsequent lift-off. After the formation of the stem cavity (120c) and the head cavity (121c), a gate metal (105) with a gate metal thickness (105t) (see FIG. 1(f)) is deposited with surrounding metal layers (105a) on the third photoresist (121). After the gate metal deposition, metal layers in the surrounding areas (105a) are removed by immersing the substrate in a solvent which dissolves the photoresist and the gate (105) is obtained (see FIG. 1(g)). In order to obtain successful lift-off, the head cavity (121c) must have reentrant cross-section so that the bottom width (121wb) is substantially larger than the top width (121wt, see FIGS. 1(e) and 1(f). Furthermore, thickness of third photoresist (121t) should be substantially larger compared to gate metal thickness (105t).